The present invention generally relates to optical fibers and more particularly, to a method of and an apparatus for coupling ends of multicore coated optical fibers each having multiple optical fibers.
Conventional methods of coupling ends of multicore coated optical fibers generally comprise a step (1) of removing a reinforcement coating layer from an end portion of each of the coated optical fibers into exposed fiber ends (glass portions) and cleaning surfaces of the exposed fiber ends, a step (2) of cutting off the exposed fiber ends to mirror surfaces, a step (3) of coupling the exposed fiber ends of the coated optical fibers through fusion bonding, etc. and a step (4) of forming a reinforcement at a joint portion of the exposed fiber ends of the multicore coated optical fibers.
In the above described known steps (1) to (4), coupling characteristics of an optical transmission path are greatly affected especially by steps (2) and (3). For example, such an undesirable phenomenon takes place that gaps between the end faces of the exposed fiber ends of the multiple optical fibers of one multicore coated optical fiber and opposite end faces of those of the other multicore coated optical fiber are scattered due to increase in amount of non-uniformity in axial positions of the end faces of the exposed fiber ends of each of the multicore coated optical fibers, thereby resulting in increase of the average coupling loss. Furthermore, in an extreme case, some of the exposed fiber ends of the multicore coated optical fibers cannot be coupled to each other. Meanwhile, in the case where one of the optical fibers is forcibly pushed to a predetermined position after its coupling with a mating optical fiber, the coupled optical fiber is sidewise deflected and therefore, is subjected to buckling so as to be brought into contact with a neighboring optical fiber, thus possibly resulting in fracture of the coupled optical fiber.
These drawbacks of the known methods mainly result from such a fact that since cutting and coupling operations of the optical fibers are performed in separate processes by using separate apparatuses, respectively, influences exerted on the optical fibers vary according to operations of the operator.
Two causes can be recited for non-uniformity in axial positions of the end faces of the exposed fiber ends during coupling of the multicore coated optical fibers. Namely, one cause is inaccuracy in cutting of the exposed fiber ends and the other cause is projection or retraction of the exposed fiber ends due to handling of the coated optical fibers, which handling is performed until a point immediately prior to coupling of the exposed fiber ends after the exposed fiber ends of the coated optical fibers have been cut off. The first cause is further classified into (A) difference in lengths of initial flaws formed on the exposed fiber ends and (B) difference in circumferential positions of the initial flaws and directions of planes for bending the exposed fiber ends. In order to cut off the exposed fiber ends, a so-called stress fracture method is usually employed in which the initial flaws are formed on the exposed fiber ends by using a blade made of hard materials such as cemented carbide, etc. and then, a bending stress or a tensile stress is applied to the exposed fiber ends so as to cause progress of fracture at the initial flaws such that fractured end faces having mirror surfaces are obtained. Accordingly, when the initial flaws are made smaller and sharper, positions of fracture of the exposed fiber ends become more uniform. On the contrary, when the initial flaws A are long as shown in FIG. 1, positions of fracture of exposed fiber ends 3 of the multicore coated optical fiber 2 are likely to be scattered due to scatter of position of the initial flaws A into an axial length B.
Meanwhile, as shown in FIG. 2, it is desirable that the initial flaw A and the bending direction f of the exposed fiber end 3 are disposed in an identical plane as closely as possible such that the fractured end face 10 of the exposed fiber end 3 is perpendicular to the axis of the exposed fiber end 3. On the other hand, as shown in FIG. 3, when the initial flaw A forms an angle with the bending direction f, such possibilities become larger that the fractured end face 10 is inclined at an angle 0 to a plane perpendicular to the axis of the exposed fiber end 3 or the fractured end face 10 becomes uneven. Furthermore, when the depth of the initial flaw A is too small, the fractured end face 10 is not finished to a mirror surface and thus, has a rough surface. In order to stably obtain the fractured end face 10 having a mirror surface, the initial flaw A of at least 2 to 3 .mu.m in depth is required to be formed on the exposed fiber end 3. Therefore, it is preferable that the initial flaw A is made as small as possible. In addition, it is most desirable that the exposed fiber end 3 is bent in a plane containing the initial flaw A and the axis of the exposed fiber end 3. However, in the case where the initial flaw A is manually formed on the exposed fiber end 3 or the exposed fiber end 3 is fractured by employing conventional cutting methods, an amount C of non-uniformity in axial positions of the fractured end faces 10 reaches at least 50 .mu.m as shown in FIG. 4.
Non-uniformity in axial positions of the fractured end faces 10 owing to the earlier said second cause, i.e., handling of the coated optical fiber 2 after cutting of the exposed fiber ends 3, is likely to increase, in amount, especially in a loosely coated multicore optical fiber in which mobility of the optical fibers relative to the coating is high. This phenomenon is divided into two cases. In one case (FIG. 5), a tapelike multicore coated optical fiber 2 is bent widthwise, so that the end faces 10 are sequentially retracted from each other in an oblique direction, thereby resulting in production of the amount C of non-uniformity in axial positions of the end faces 10. In the other case (FIG. 6), only one 8 of the optical fibers is curved in the tapelike multicore coated optical fiber 2, so that the exposed fiber end 3 of the optical fiber 8 is projected by the distance C or retracted from those of the remaining optical fibers by axially stretching out the optical fiber 8. The amount C of non-uniformity in axial positions of the fractured end faces 10 in these two cases of FIGS. 5 and 6 sometimes reaches as large a value as 100 .mu.m.